EXPERIMENT NO. 02
AM DEMODULATION
Lab Objective
To implement and analyze Amplitude Demodulation on DATEX trainer.
Introduction
The process of separating or extracting the modulation from a signal is called demodulation or
detection. For amplitude modulation, the process of demodulation or detection can be
accomplished very simply using a diode, or it may be achieved in other ways that provide more
effective demodulation of the waveform.
Theory
If you've completed Experiment AM Modulation, then you've seen what happens when a 2kHz
sinewave is used to amplitude modulate a carrier to produce an AM signal. Importantly, you would
have seen a key characteristic of an AM signal - its envelopes are the same shape as the message
(though the lower envelope is inverted).
Recovering the original message from a modulated carrier is called demodulation and this is the
main purpose of communications and telecommunications receivers. The circuit that is widely
used to demodulate AM signals is called an envelope detector. The block diagram of an envelope
detector is shown in Figure 1 below
As you can see, the rectifier stage chops the AM signal in half letting only one of its envelopes
through (the upper envelope in this case but the lower envelope is just as good). This signal is fed
to an RC LPF which tracks the peaks of its input. When the input to the RC LPF is a rectified AM
signal, it tracks the signal's envelope. Importantly, as the envelope is the same shape as the
message, the RC LPF's output voltage is also the same shape as the message and so the AM signal
is demodulated.
A limitation of envelope detector shown in Figure 1 is that it cannot accurately recover the message
from over-modulated AM signals. To explain, recall that when an AM carrier is over modulated
the signal's envelope is no-longer the same shape as the original message. Instead, the envelope is
distorted and so, by definition, this means that the envelope detector must produce a distorted
version of the message.
LAB SESSION
The experiment
For this experiment you'll use the Emona DATEx to generate an AM signal by implementing its
mathematical model. Then you'll set-up an envelope detector using the Rectifier and RC LPF on
the trainer's Utilities module.
Once done, you'll connect the AM signal to the envelope detector's input and compare the
demodulated output to the original message and the AM signal's envelope. You'll also observe the
effect that an over-modulated AM signal has on the envelope detector's output.
Finally, if time permits, you'll demodulate the AM signal by implementing by multiplying it with
a local carrier instead of using an envelope detector.
Equipment
• Personal computer with appropriate software installed
• NI ELVIS II plus USB cable and power pack
• Emona DATEx experimental add-in module
• Two BNC to 2mm banana-plug leads
• Assorted 2mm banana-plug patch leads
• One set of headphones (stereo)
Experimental Procedure
Part A - Setting up the AM modulator
To experiment with AM demodulation, you'll need an AM signal. The first part of the experiment
gets you to set one up.
1. Ensure that the NI ELVIS II power switch at the back of the unit is off.
2. Carefully plug the Emona DATEx experimental add-in module into the NI ELVIS II.
3. Set the Control Mode switch on the DATEx module (top right corner) to PC Control.
4. Connect the NI ELVIS II to the PC using the USB cable.
Note: This may already have been done for you.
5. Turn on the NI ELVIS II power switch at the rear of the unit then turn on its
Prototyping Board Power switch at the top right corner near the power indicator.
6. Turn on the PC and let it boot-up.
7. Launch the NI ELVISmx software.
8. Launch and run the NI ELVIS II Variable Power Supplies VI.
9. Adjust the Variable Power Supplies negative output Voltage control for an output of about -6V
(the exact value is not critical).
10. Launch and run the NI ELVIS II DMM VI.
11. Set up the DMM VI for measuring DC voltages.
12. Launch the DATEx soft front-panel (SFP) and check that you have soft control over the
DATEx board.
13. Locate the Adder module on the DATEx SFP and turn its soft G and g controls fully anticlockwise.
14. Connect the set-up shown in Figure 2 below.
Oscilloscope Output
Result
15. Connect the Adder module's output to the DMM and adjust the module's soft g control to obtain
a 1V DC output.
Note 1: Remember that you must also connect the DMM's COM input to a ground terminal
on the Emona DATEx.
Note 2: Remember also that you can use the keyboards tab and arrow keys for fine
adjustment of DATEx soft controls.
16. Disconnect the DMM and close its VI.
17. Launch and run the NI ELVIS II Oscilloscope VI.
18. Set up the scope per the procedure in Experiment 1 with the following changes:
• Channel 0 Coupling control to the DC position instead of AC
• Channel 0 Scale control to the 500mV/div position instead of 1V/div
• Trigger Level control to the 1V position instead of 0V
19. Adjust the scope's Time base control to view only two or so cycles of the message signal.
20. Adjust the Adder module's soft G control to obtain a 1Vp-p sinewave.
21. Activate the scope's Channel 1 input to view both the message and the modulated carrier.
Self check: If the scope's Scale control for Channel 1 is set to the 1V/div position, the scope
should now display an AM signal with envelopes that are the same shape and size as the
message. If not, close all windows, check your wiring then repeat the process starting from
Step 7.
The set-up in Figure 2 on the previous page can be represented by the block diagram in Figure 3
below. It generates a 1OOkHz carrier that is amplitude modulated by a 2kHz sinewave message.
Part B - Recovering the message using an envelope detector
22. Modify the set-up as shown in Figure 4 below.
Remember: Dotted lines show leads already in place.
The additions to the set-up can be represented by the block diagram in Figure 5 below. As you can
see, it's the envelope detector explained in the preliminary discussion.
Oscilloscope Output after Rectifier
Result
23. Adjust the scope's Scale and Time base controls to appropriate settings for the signals.
24. Draw the two waveforms to scale in the space provided below leaving room to draw a third
waveform.
Tip: Draw the message signal in the upper third of the graph and the rectified AM signal in the
middle third.
25. Disconnect the scope's Channel 1 input from the Rectifier's output and connect it to the
RC LPF's output instead.
26. Draw the demodulated AM signal to scale in the space that you left on the graph paper.
Oscilloscope Output after LPF
Result
LAB REPORT
Discussion of Results
Conclusion /Summary
References (IEEE Method)